Turbulent Drag Reduction and Degradation of DNA

Turbulent drag reduction induced by � -DNA is studied. The double-stranded DNA is found to be a good drag reducer when compared with the other normal linear polymers. However, this drag reducing power disappears when the DNA denatures to form two single-strand molecules. Mechanical degradation of DNA is also different from that of the normal linear-chain polymers: DNA is always cut in half by the turbulence. Our results suggest that the mechanism for turbulent degradation of DNA is different from that of the normal flexible long-chain polymers. microscales of the turbulent flow. Since the length of the polymers traditionally used in DR studies are much smaller than the microscales of the turbulence, the inter- actions between the polymer and the flow might be quite different. Second, a large change in molecular properties of DNA can be induced by a change in pH or temperature. At the appropriate pH or temperature, a DNA molecule will change from its double-stranded (ds) natural state to its denatured state of two single-strand (ss) molecules. This configurational change might help to probe the details of the DR phenomenon. Hand and Williams (8) reported in a brief note that the natural state of DNA is preferable to the denatured state for maximum drag reduction in an experiment using calf- thymus DNA with pH control. But no explanation was given. In our experiments, we find that the same phenom- ena can be observed in our system, and this change in DR behavior of the DNA can be attributed to the change in mechanical properties of the two different states of DNA. It is found that the drag reducing power of the molecule is controlled mainly by the stiffness of the molecule. Dif- ferent from other linear polymers, our MMD results in- dicate that dsDNA molecules are always cut exactly in half by the turbulence even when the microscale of the turbu- lence is smaller than the size of the DNA. This simple degradation suggests that the dynamics of degradation of dsDNA is different from those of normal linear chain polymers. It is possible that the difference is due to the long persistence length of the double helix. Furthermore, the simple degradation and monodispersity of DNA allow the construction of a simple degradation model. From this model, various physical parameters not easily accessible or reported before can be estimated.